A current goal of coal technology is to separate pyrite and coal from one another to prepare a coal of the greatest possible purity.
German Patent No. 744,805 discloses a process for electrostatic separation of mixtures of coal and ore by applying an oily wetting agent to increase the surface resistance of at least a portion of the mixture. The wetting agent also has the purpose of increasing the adhesion of the wetted separable material to the wetted electrode. The oil used is not further described as to its chemical composition. The separation of the components relies on the conductivity difference between the components which is produced or enhanced by the wetting agent and not on the contact electric charge of the components as in the process of this invention.
The process of this invention uses a different method of solving the problem which has significant technical advantages.
U.S. Pat. No. 3,073,447 discloses an electrostatic beneficiation of crude potassium salts containing langbeinite into a sylvine concentrate, using known potassium chloride selective reagents, and a langbeinite concentrate using langbeinite-selective reagents at a temperature between 40.degree. and 150.degree.C. Table 1, line 15, shows that sylvine is also recovered by separation using glycerine esters of fatty acids at 60.degree.C., a method which, in comparison to other potassium chloride specific methods, shows a very poor yield of K.sub.2 O. According to the process of this invention, no glycerine esters of fatty acids are used but rather selected glycerides of stearic, palmitic, oleic, linoleic, and linolenic acids.
In contrast with the above technology, the process of this invention uses other conditioning substances and produces a surprisingly selective separation. A coal concentrate containing over 80% pure coal is attained with a yield of at least 80%.
Various authors have addressed themselves in "Aufbereitungs-Technik" (No. 4/1970, pages 207-220) to the problem of separation of pyrite from coal dust in electric and magnetic fields.
The separation was undertaken in the electrodynamic field of a cylindrical separator at a temperature between 14.degree. and 34.degree.C. and relative humidity of 20-90%. As conditioning substances for altering the conductivity, HNO.sub.3, H.sub.2 SO.sub.4, H.sub.2 O.sub.2, and KM.sub.g PO.sub.4 were employed. The report states concerning the separation results:
"From a complete review of all the results, which were obtained in numerous experiments with the electrodynamic cylindrical separator, it was abandoned, since the separation results in all experiments -- even in the case of the most favorable assumptions about raw materials -- were unsatisfactory."
These investigations did not suggest the conditioning means used in the process of this invention. Rather, the prior technology proceeded in an entirely different direction in which separations were made using differences between conductors and non-conductors and not by means of contact electric charges. These processes have the disadvantage of a low specific output of the cylindrical separator, since the electric field for charging and separation must come in contact with each mineral particle at the charging electrode for the purpose of charging and charge equalization.